(a) Field of the Invention
The present invention relates to a method and apparatus for modulating a baseband signal in beam space multi-input multi-output (MIMO).
(b) Description of the Related Art
Nowadays, in various communication technologies, a MIMO technique is adapted. Such a MIMO technique has a merit that can increase a data rate and maximize frequency efficiency. A Wibro system and a 3GPP cellular communication system as well as IEEE 802.16 and IEEE 802.20, which are portable Internet systems, adapt a MIMO technique.
A transmitting performance in such a MIMO communication system generally increases proportional to the number of antennas. Therefore, in order to maximize a MIMO performance, the number of antennas should be increased and thus the number of radio frequency (RF) chains also increases. When the antenna number increases, implementation complexity increases and a size of a system increases, and thus there is a drawback that the antenna number cannot be increased much. In order to deviate from such a restriction, nowadays, research for achieving MIMO performance using one RF chain or a small number of RF chains has been performed. As a representative example thereof, there is beam space MIMO technology using an electrical steering parasitic array radiation (ESPAR) antenna or a load modulation antenna.
Such beam space MIMO technology is different from general MIMO technology in an antenna/RF aspect and a baseband aspect.
First, in an antenna/RF aspect, beam space MIMO technology will be described. General MIMO technology forms an antenna using a plurality of active antenna elements, but beam space MIMO technology forms an antenna using one active antenna element or a small number of active antenna elements and a plurality of parasitic antennas elements. Such beam space MIMO technology can obtain an increasing effect of the antenna number through a plurality of parasitic antennas elements and reduce a distance between a plurality of parasitic antenna elements. Further, because beam space MIO technology uses one RF chain or a small number of RF chains, an RF portion may be implemented with no complex portion and a small size.
In a baseband aspect, beam space MIMO technology will be described. General MIMO technology uses a plurality of active antenna elements and emits a baseband signal that is modulated on each active antenna basis. Accordingly, by a phase difference and a magnitude difference due to a path to which each signal is transferred, a phase and a magnitude of a signal that is finally emitted through an active antenna are determined. Beam space MIMO technology has a form in which a signal passes on each path basis, and a phase and a magnitude of each signal that is emitted from an antenna are affected by RF impairment such as phase noise of a local oscillator or IQ imbalance of a transmitting path. By measuring such RF impairment, in a baseband aspect, when compensation is performed or when an RF is formed, a system may be formed in consideration of such a specification. However, because such RF impairment (i.e., IQ imbalance or phase noise) is not rapidly changed but has an almost constant value, RF impairment may be compensated with a long cycle.
In beam space MIMO technology, when decomposing an entire antenna beam pattern that is generated by one active antenna or a small number of active antennas and a plurality of parasitic antennas, a plurality of orthogonal beams are generated. The baseband signal is mapped to such a plurality of orthogonal beams. A current value flowing to a plurality of parasitic antennas is changed by a baseband signal and an impedance value of a plurality of parasitic antennas, and thus an emission signal that is mapped to the beam is finally generated. A phase and a magnitude of a signal that is finally emitted by the current value are determined. That is, by changing a load value (impedance value of a plurality of parasitic antennas), a phase and a magnitude of a finally emitted signal are changed and thus a desired emission signal is generated.
A plurality of parasitic antennas are implemented by a variable impedance element, and a value of the variable impedance element is affected by a signal occurring in a baseband. That is, whenever a value of a signal occurring in a baseband changes, a load value should be changed. Therefore, whenever a baseband signal changes, tuning is required, and it is necessary that beam space MIMO technology tunes with a cycle faster than that of general MIMO technology.
In other words, in beam space MIMO, when symbols of each baseband signal are different, a phase difference (change) and a magnitude difference (change) are not constantly emitted in each symbol. Thereby, a phase and a magnitude of a finally emitted signal do not maintain a phase and a magnitude of a baseband. When measuring a phase and a magnitude of a channel with a reference signal, a portion in which a reference signal does not exist may have a phase and a magnitude that are different from those of a reference signal, and thus a problem may occur that normal channel estimation is impossible.